1. Field of the Invention
This invention relates to a polyurethane prepolymer and the resulting flexible foam that can be made from the prepolymer when it is mixed with water.
2. Description of Previously Published Art
Flexible polyurethane foams made with TDI (toluene diisocyanate) have been manufactured for many years, especially for cushion and mattress applications. However, for hydrophilic foams used in medical or personal care applications it is desirable to replace the TDI in the foams with MDI (methylene diphenyl isocyanate) because of the high vapor pressure and relatively high toxicity of TDI which requires special precautionary measures during processing and use. Furthermore, TDI-based foams can be weakened by hydrolysis during sterilization or storage in a wet package. For example, TDI-based hydrophilic foams can liquify after a few cycles in a steam autoclave at 120.degree. C. TDI-based hydrophilic foams also swell excessively when wet such as on the order of more than 100% by volume.
Conventional polyurethane foams have been made from MDI. These foams are rigid or semi-rigid because MDI imparts crystallinity. In British Pat. No. 874,430, flexible polyurethane foams are produced by reaction of polyether polyols with at least two hydroxyl groups and a polyisocyanate mixture consisting of diarylmethane diisocyanates and 5 to 10 percent by weight of a polyisocyanate having a functionality greater than 2 in the presence of a small amount of water. A catalyst can be used in optional embodiments. These foams have the disadvantages that they are not hydrophilic and are not made with sufficient quantities of water to allow transport of large amounts of fibers, fillers, antiseptics, or other water-dispersible components into foams used in medical or personal care applications. The term hydrophilic as used herein means that the foam product is able to absorb 15-20 times its weight of water. A further disadvantage is that in the case of the optional catalyst there can remain catalyst residue which is not desirable.
In U.S. Pat. No. 4,237,240, flexible MDI-based foams with high load-bearing and high energy-absorption capacity are made by reaction of diphenylmethane diisocyanates with polyester polyols or mixtures of polyester polyols and polyether polyols with a polyester polyol content of at least 60 percent by weight of the polyol mixture, and small amounts of water. As set forth in the claims, a catalyst is employed. These foams have the same drawbacks as those of the above-described British Pat. No. 874,430 including the undesirable catalyst residues in the foam and in addition they require the use of the more expensive polyester polyols.
In British Pat. No. 1,209,058, flexible hydrophilic polyurethane foams can be made by reacting a polyisocyanate with polyether polyols which contain at least 10% by weight of a block copolymer of ethylene oxide capped with propylene oxide to obtain hydrolytic stability. The method requires using at least one divalent tin salt of a fatty acid and/or at least one tertiary amine as a catalyst. The foam products made by this method, while being hydrophilic, have the drawback of being made with only small amounts of water as well as requiring the use of the block copolymers. Moreover, there is no teaching of the use of MDI, which is hydrophobic to make hydrophilic foam products and the resulting foam will contain undesirable catalyst residues.
3. Objects of the Invention
It is an object of this invention to provide an improved, flexible polyurethane foam which is made from a prepolymer system containing MDI as the only isocyanate source.
It is a further object of this invention to provide an improved, flexible polyurethane foam which can contain large amounts of fibers, fillers, antiseptics or other water-dispersible components.
It is a further object to produce a flexible, hydrophilic foam which is white in color so that it can be used in medical and health care applications.
It is a further object to produce a polyurethane prepolymer having MDI and its derivative as the only isocyanate source which can be stored for an indefinite period of time and yet when mixed with an approximately equal amount of water, which can be used as a carrier for water-dispersible ingredients, will form a hydrophilic foam product.
It is a further object to produce a polyurethane prepolymer having MDI and is derivative as the only isocyanate source which has viscosity stability over time.
It is also an object to produce a polyurethane prepolymer having MDI and its derivatives as the only isocyanate source which has less than 50% by weight of the isocyanate and which will produce a flexible foam.
These and further objects will become apparent as the description of the invention proceeds.